Alumina bonded abrasive for cast iron

ABSTRACT

This invention concerns the discovery that sol-gel process allumina abrasive grains provides superior performance in bonded abrasives for grinding cast iron. In a preferred form it is disclosed that bonded abrasives of sol-gel process alumina abrasives in combination with other abrasive grains such as those of silicon carbide and fused alumina also provide superior grinding of cast iron.

This is a continuation of application Ser. No. 07/392,825 filed Jul. 27,1989, Which is a continuation of 07/245,589, filed Sep. 19, 1988 (nowU.S. Pat. No. 4,883,501), which is a continuation of 06/666,131, filedOct. 30, 1984 (now U.S. Pat. No. 4,800,685), which is a continuation of06/615,663, filed May 31, 1984 (now abandoned), which is a continuationof 06/536,830, filed Sep. 30, 1983 (now abandoned), which is acontinuation of 06/331/442, filed Dec. 15, 1981 (now abandoned).

TECHNICAL FIELD

This invention relates to bonded abrasive materials and methods ofabrasion utilizing abrasive materials, and more particularly relates tothe grinding of cast iron.

BACKGROUND ART

The grinding of cast iron generally requires bonded abrasive articles,usually wheels, which are capable of removing large amounts of metal.Preferably, the wheels have long life for more efficient grinding andlower cost. Bonded abrasive wheels are used for trimming and cleaning ofcastings of cast iron; this use is commonly called snagging. In thesnagging art, it has been known that bonded abrasive grinding wheelsemploying aluminum oxide as the sole abrasive have not been favored. Itis believed that grains of standard fused aluminum oxide are tooresistant to wear or breakage and, in use, the grains tend to becomedull instead of breaking to expose new grinding surfaces. The dullsurface of the grain results in low metal removal rates and generationof excessive frictional heat.

Therefore, in grinding, i.e., snagging of cast iron, the preferredabrasive historically has been a combination of silicon carbide andaluminum oxide. It is believed that the silicon carbide, being morefriable than aluminum oxide, causes the wheel to wear at an acceleratedrate and constantly expose new, sharper grinding surfaces. A more recentdevelopment has been the use of a co-fused zirconia-alumina abrasive, asdescribed in U.S. Pat. No. 3,181,939. This material employs a particularcrystalline structure to produce an even rate of wear and creation ofnew grinding surfaces.

While the zirconia abrasives which comprise a combination of aluminawith about 35 to 50% zirconia have performed satisfactorily, their highcost has created a desire for a less expensive abrasive. Further,alumina zirconia abrasives are more dense than other abrasives and therewould be an advantage in a lighter abrasive which would provide lessrotational stress in abrasive wheels and less operator fatigue duringhand grinding.

Recently in European Patent Publication No. 0 024 099, published Feb.25, 1981 a sol-gel alumina abrasive grain is disclosed. This grain issuggested for use in bonded abrasives, but examples and tests allinvolve use with coated abrasives.

U.S. Patent Application Ser. No. 267,495, filed May 27, 1981 in thenames of Robert Seider and Alvin Gerk also discloses sol-gel abrasives,and suggests use in either coated or bonded applications.

DISCLOSURE OF THE INVENTION

It is an object of this invention to overcome disadvantages of priorbonded abrasive articles for grinding cast iron.

A further object of this invention is to produce an improved abrasivefor the grinding of cast iron.

Another object of this invention is to form high performancelight-weight grinding wheels.

A further object of this invention is to form grinding wheels whichgrind cast iron at higher rates of metal removal than prior abrasives.

Another further object of this invention is to form grinding wheels thatcut cast iron with less pressure.

A still further object of this invention is to form a bonded abrasivefor grinding cast iron with a longer life than achieved by priorabrasives.

These and other advantages of the invention are achieved by utilizationof a sol-gel process abrasive grain in a bonded abrasive article usedfor grinding cast iron.

It has been found that, when grinding cast iron, sol-gel abrasivesprovide far superior performance than ordinary fused alumina abrasives.In fact, surprisingly, the performance is equal or superior to that offused alumina zirconia. Surprisingly, sol-gel alumina abrasives aresofter than fused alumina abrasives but nevertheless do not dull easilyin bonded abrasive use. Ordinary fused alumina grain becomes dull ratherthan breaking and the grinding becomes slow and ineffective.Zirconia-alumina abrasives also are softer than fused alumina.

BEST MODE FOR CARRYING OUT THE INVENTION

The system of the invention may utilize any size or shape bondedabrasive article which is useful for grinding cast iron. For example,bonded abrasive articles may be in the form of a cup wheel generallybetween 4 and 6 inches in diameter, or may be in the form of a snaggingwheel of up to 36 inches in diameter, or may also be in the form of adepressed center wheel of up to about 9 inches in diameter. Aparticularly preferred use of the invention is in the larger snaggingwheels. It is believed that lighter weight of the alumina grain allowshigher speed of the wheel, and therefore higher cutting rates, thanpossible with heavier zirconia alumina grains.

The invention may be practiced with any matrix or bonding system. Apreferred bonding system is a phenolic resin bond. This is aparticularly desirable bond for wheels used for grinding cast iron as itis not brittle and does not break under the abuse to which the bondedabrasive is subjected during snagging. However, other bonding systemssuch as other resins, rubber, shellac or vitrified bonds may beutilized, particularly when more precise grinding than snagging is beingperformed.

It is known in the formation of bonded abrasive articles that mixturesof grains may be utilized. In some cases, mixtures are utilized toprovide an improvement in performance, such as the utilization ofsilicon carbide in combination with fused aluminum oxide for snagging ofiron. In the case of the utilization of sol-gel formed grains insnagging of cast iron, it may be desirable to blend the sol-gel grainswith a lower cost abrasive grain such as conventional silicon carbide.In this instance, however, the blending would be to lower cost of thebonded abrasive at a trade-off of slightly less performance. Any ratioof conventional silicon carbide grain to sol-gel abrasive grain may beutilized in this invention. A preferred blend is between about 50 andabout 80% sol-gel alumina abrasive by weight of total grain weight ofthe abrasive for good metal removal at a reasonable cost. An optimumblend would include about 70% by weight of total grain weight sol-gelalumina abrasive grain for a bonded abrasive article, and this articlewould perform almost as well as 100% sol-gel alumina grain but at lowercost. However it is within the scope of the invention to blendconventional silicon carbide grain or other abrasive grain in a range ofbetween 0 and about 70% by weight of total grain depending on theparticular utilization and cost desired.

It is also within the scope of the invention to form bonded abrasivearticles of blends of sol-gel alumina grain and the conventional fusedalumina abrasive grains. Such blends have been found to give goodperformance with less use of the sol-gel abrasives and some weightsavings over wheels containing zirconia-alumina abrasive. Surprisingly,articles having blends of fused alumina and sol-gel alumina have beenfound in some instances to give superior performance to bonded abrasivearticles containing only sol-gel abrasives or zirconia abrasives; noteExamples 7, 8 and 9.

The blends of sol-gel alumina grain and fused alumina grain may be ofany percentage blend which gives the desired performance characteristicsat a particular manufacturing cost. Typically, the blended bondedabrasive article would contain sol-gel alumina grain greater than about30% of the grain weight. A preferred range is between about 30 and about70% of total grain weight of the sol-gel abrasive for best performance.About 70% by grain weight of sol-gel alumina grain appears to giveoptimum performance in view of results showing that metal removal rateis high and wear of the abrasive article is low.

In the bonded abrasives of the invention for grinding cast iron, thesol-gel grain may be of any size suitable for the use intended.Generally, grit size of between about 10 and about 30 grit is utilizedfor snagging wheels although for some uses grit of up to about 4 gritmay be utilized. Other grits such as for use with vitreous bonds forfiner work may be between about 46 and about 100 grit. The term castiron as used herein refers to as-cast gray cast iron and to malleablecast iron formed by annealing white cast iron. It also refers to nodularcast iron where the matrix is a mixture of pearlite and ferrite and issubstantially free of massive primary carbides.

The sol-gel abrasive utilized in the invention may be any sol-gel formedceramic composition which has been demonstrated to have abrasivequalities. Such sol-gel abrasives are formed by a technique which hasbeen referred to as chemical processing as opposed to the conventionaltechnique for fused alumina abrasives in which alumina material isbrought to a molten temperature and then cooled. The cooled material isthen crushed and graded by size to form grinding materials. In contrast,the chemical formation involves formation of a sol which is gelled,dried, crushed and then heated to sintering temperature. Typical ofsol-gel grains are those disclosed in European Patent ApplicationPublication No. 0 024 099, published on Feb. 25, 1981, and in U.S.application Ser. No. 267,495, filed on May 27, 1981 by R. Seider and A.Gerk. Grains formed by the techniques of Seider and Gerk are preferredbecause of low cost and good abrasive properties. The disclosure of U.S.Ser. No. 267,495 is hereby incorporated by reference for its teaching ofgrains suitable for the invention.

The term performance when used herein in reference to bonded abrasivearticles refers to either or both of two measurements. One measurementis "grinding ratio" (gained by dividing total metal removed by totalwheel wear) which relates to the total amount of metal a bonded abrasivearticle will remove before it wears out. The second measurement is"metal removal rate" which is the speed of metal removal without regardto how quickly the bonded abrasive article is wearing away. Generally,how important each factor is depends on the use and customer. If largeamounts of metal need to be removed quickly or if labor costs are high,then metal removal rate is most important. The retail market such as tothe homeowner may value grinding ratio more highly as an indication thatthe bonded abrasive wheel will last a longer time.

The following Examples illustrate the system of the invention incomparison with prior abrasives for cast iron. All formulations are byweight unless otherwise indicated. Weights are in grams unless otherwiseindicated.

EXAMPLES 1-5

The following procedure was used to produce grinding wheels for testingsol-gel abrasive performance:

Five Example abrasive formulations were manufactured, with compositionsas shown in Table 1. With regard to the raw materials, the grit sizedesignations and the terms "black silicon carbide", "brown aluminumoxide", and "powdered fluorspar" (a fine grind fluorspar of 95% purityused as a grinding aid) refer to materials commonly used in theindustry. A typical black silicon carbide is C6 silicon carbide and atypical brown aluminum oxide is C35 aluminum oxide both available fromThe Carborundum Company, Niagara Falls, N.Y. "Zirconia abrasive" refersto an abrasive mixture of about 40% zirconia and about 60% alumina,fused and cooled to form a eutectic crystalline structure available fromNorton Company, Worcester, Mass. Examples 1, 5 and 4 are controls.

The liquid resole used was Varcum 8121; the powdered resin was Varcum7909; both are phenolic resins and are available from Varcum ChemicalCompany Division of Reichold Chemical, Niagara Falls, N.Y.

Mixes were made with each formulation by placing the abrasive grain intoa mixer, adding the liquid resin, and mixing until a uniform coating wasachieved. The remaining ingredients were blended thoroughly, then addedslowly to the wetted grain in such a manner that all the powder waspicked up by the abrasive grains. In this way, granular free-flowingmixes free of loose powder were achieved.

Abrasive products of the type known as Type 11 cup wheels, 6×2×5/8-11,were made with each of the test mixes. This is a cup wheel 6 inches ingreatest diameter, 2 inches thick and has an arbor 5/8 inch in diameter.Into a steel mold of appropriate size and shape was placed a threadedmetal bushing, followed by a specified amount of abrasive mixture whichwas then evenly distributed within the mold. The mold was covered,placed in a hydraulic press, and pressed to apply 3000 psi of pressureto the abrasive wheel. After pressing, the mold was disassembled and thepressed abrasive article measured and checked for density. In order toproduce articles with comparable volumetric percentages of abrasive,bond, and porosity, wheels of formula 1 and 2 were pressed to a densityof 2.34 grams/cc, those of formula 3 and 5 to 2.54 grams/cc, and thoseof formula 4 to 2.79 grams/cc.

The resulting uncured abrasive products were bedded in sand and cured inan oven cycle of:

heat from 80°-350° F. 26 hours

hold at 350° F. 9 hours.

After cooling, the grinding wheels were cleaned and speeded to 1.5 timestheir operating speed before being tested.

Three wheels of each formula were tested for grinding performance usingthe following test procedure:

A test wheel was mounted on a Chicago Pneumatic Grinder, model CP3490,and applied to a 12 inch round plate of Class 35 cast iron for a 15minute grinding period by an individual knowledgeable in manual grindingprocedures. The weights of both the grinding wheel and cast iron platewere measured before and after the test, and the differences in weightwere calculated as "total metal removed" and "total wheel wear". A thirdvalue, called "grinding ratio" was calculated by dividing total metalremoved by total wheel wear. This ratio is commonly used in the industryto compare wheel performance;. a larger value generally represents abetter performance.

Average values for the test wheels were found to be as shown in Table 2.It may readily be noted that as sol-gel alumina abrasive replaces thestandard alumina and silicon carbide in the standard formula of Example1, then both the metal removal and the grinding ratio are increased. Itmay further be noted that a formula consisting of all sol-gel aluminaabrasive removes metal at 90% the rate of an equivalent formulautilizing zirconia abrasive, and at a significantly better grindingratio. The increased grinding ratio results in a significantly betterwheel in commercial value.

Example 5 illustrates the general unsuitability of a formula containing100% fused alumina abrasive. Metal removal rate is much lower than theother Examples, indicating a wheel which would provide low productivityin use.

                  TABLE 1                                                         ______________________________________                                        COMPOSITION OF ABRASIVE WHEEL                                                 FORMULATIONS IN PARTS BY WEIGHT                                                           (Control)            (Control)                                                                            (Control)                             Material Formula                                                                          Ex. 1    Ex. 2  Ex. 3                                                                              Ex. 4  Ex. 5                                 ______________________________________                                        14 grit black silicon                                                                     36.8     36.8   --   --     --                                    carbide                                                                       14 grit brown                                                                             --       --     --   --     42.0                                  aluminum oxide                                                                14 grit sol-gel                                                                           --       --     42.0 --     --                                    abrasive                                                                      14 grit zirconia                                                                          --       --     --   42.7   --                                    abrasive                                                                      16 grit brown                                                                             22.8     --     --   --     21.0                                  aluminum oxide                                                                16 grit sol-gel                                                                           --       22.8   21.0 --     --                                    abrasive                                                                      16 grit zirconia                                                                          --       --     --   21.4   --                                    abrasive                                                                      20 grit brown                                                                             22.8     --     --   --     21.0                                  aluminum oxide                                                                20 grit sol-gel                                                                           --       22.8   21.0 --     --                                    abrasive                                                                      20 grit 43% zirconia                                                                      --       --     --   21.3   --                                    abrasive (made in                                                             accordance with                                                               U.S. Pat.                                                                     No. 3,891,408)                                                                Liquid phenolic                                                                           2.8      2.8    2.8  2.6    2.8                                   resole                                                                        Powdered fluorspar                                                                        5.6      5.6    4.9  4.5    4.9                                   (grinding aid)                                                                600 grit black                                                                            2.0      2.0    2.0  1.8    2.0                                   silicon carbide                                                               Powdered phenolic                                                                         7.2      7.2    6.3  5.7    6.3                                   novolac                                                                       ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        GRINDING PERFORMANCE OF TYPE 11 CUP                                           WHEELS - AVERAGES OF THREE WHEELS                                             (OF EACH EXAMPLE)                                                                    Total Metal                                                                              Metal                                                              Removed,   Removal    Total Wheel                                                                            Grinding                                Example                                                                              Grams      Rate, gm/min                                                                             Wear, Grams                                                                            Ratio                                   ______________________________________                                        1       728       48.5       56.3     13.3                                    (Control)                                                                     2      1189       79.3       69.5     17.1                                    3      1667       111.1      33.6     49.6                                    4      1846       123.1      61.9     29.9                                    (Zirconia                                                                     Abrasive                                                                      Control)                                                                      5       579       38.6       10.2     61.4                                    (Control)                                                                     ______________________________________                                    

EXAMPLES 6-10

To test a different range of grinding parameters, example formulationswere manufactured with compositions as shown in Table 3. Materials usedwere identical to those defined in Examples 1-5, unless otherwiseindicated.

Mixing was performed in the same manner as described in the precedingExamples.

Abrasive products of the type known as Type 27 depressed center wheels,7×1/4×7/8, were made with each of the test mixes. This is a wheel 7inches in diameter, 1/4 inch thick, with a 7/8 inch arbor hole, having adepressed area in the hub to hold the mounting nut, which allows thewheel to be used for close grinding without interference of the mountingnut on the workpiece. Into a steel mold of appropriate size and shapewas placed a 7 inch disc of tissue paper, followed by a 7 inch disc ofphenolic resin coated fiberglass mesh. A specified amount of abrasivemixture was added and evenly distributed within the mold, followed by asecond 7 inch disc of phenolic resin coated fiberglass mesh. A secondspecified amount of abrasive mix was then added and evenly distributedwithin the mold. The mold was covered, placed in a hydraulic press, andpressed so as to apply 3,000 psi of pressure to the abrasive wheel.After pressing, the mold was disassembled and the abrasive articlemeasured and checked for density. In order to produced articles withcomparable volumetric percentages of abrasive, bond, and porosity,wheels of Examples 6-9 were pressed to a density of 2.65 gm/cc., whilethose of Example 10 were pressed to a density of 2.83 gm/cc.

(The phenolic resin coated fiberglass mesh discs referred to areproduced in a variety of weave styles and strengths, which may beselected by those familiar with the art of making abrasive products toprovide specific desired properties to a grinding wheel. The discs usedin the Example are identified as "707 CRX" material, as provided by theEli Sandman Co., Worcester, Mass.; however, the scope of the inventionshould not be interpreted as limited to products using this specificmaterial.)

The resulting uncured abrasive products were placed between aluminumbatts, then cured in the same oven cycle described previously. Aftercooling, the wheels were speed tested at 1.5 times their operating speedbefore being tested.

Three wheels of each formulation were tested for grinding performanceusing the following test procedure:

A test wheel was mounted on a Bosch electric grinder, Model 1321, andapplied to a 12 inch round plate of Class 35 cast iron for a 20 minutegrinding period by an individual knowledgeable in manual grindingprocedures. The weights of both wheel and plate were measured bothbefore and after the grinding test. "Total metal removed", "total wheelwear" and "grinding ratio" were calculated as previously described.Average values for the various test wheels were found to be as shown inTable 4.

It may be noted from these examples that as sol-gel abrasive replacesbrown fused alumina, with all else remaining unchanged, a steadyincrease in rate of metal removal occurs. It may further be noted thatblends of sol-gel abrasive and brown aluminum oxide produce abrasivewheels whose grinding ratios exceed those achievable with eithermaterial used alone. By proper formulation, performance comparable orsuperior to that of zirconia abrasive may be achieved.

                  TABLE 3                                                         ______________________________________                                        COMPOSITION OF TP27 ABRASIVE WHEEL                                            FORMULATIONS IN PARTS BY WEIGHT                                                            (Control)                  (Control)                             Material Formula                                                                           Ex. 6    Ex. 7  Ex. 8 Ex. 9                                                                              Ex. 10                                ______________________________________                                        24 grit brown                                                                              73.0     51.1   21.9  --   --                                    aluminum oxide                                                                24 grit sol-gel                                                                            --       21.9   51.1  73.0 --                                    abrasive                                                                      24 grit zirconia                                                                           --       --     --    --   73.5                                  abrasive                                                                      Liquid phenolic                                                                             3.5      3.5    3.5   3.5  3.4                                  resole                                                                        Powdered fluorspar                                                                         13.0     13.0   13.0  13.0 12.0                                  (grinding aid)                                                                Powdered phenolic                                                                          10.5     10.5   10.5  10.5  9.1                                  novolac                                                                       ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        GRINDING PERFORMANCE OF TYPE 27                                               WHEELS - AVERAGES OF THREE                                                    WHEELS (OF EACH EXAMPLE)                                                                 Total     Metal    Total                                                      Metal     Removal  Wheel                                                      Removed,  Rate,    Wear,   Grinding                                Example    Grams     gm/min   Grams   Ratio                                   ______________________________________                                         6 (Control)                                                                              755      37.8     7.3      98.8                                    7 30% Sol-Gel                                                                           1007      50.4     10.3    107.0                                    8 70% Sol-Gel                                                                           1223      61.2     8.3     149.5                                    9 100% Sol-Gel                                                                          1405      70.3     13.3    108.2                                   10 (Zirconia                                                                             1361      68.1     9.3     148.7                                   Abrasive)                                                                     ______________________________________                                    

The above Examples are intended to be illustrative and not exhaustive ofthe sol-gel bonded abrasive devices of the invention. For instance,other resins than phenolic resins, could be utilized. The bondedabrasives could be formed with rubber or a vitreous binder.

The invention may be utilized with suitable modifications within thestate of the art in other types of bonded abrasives that may be utilizedin abrasion of cast iron. For instance, the bonded abrasive could beused in small vitreous stones for fine work or in bonded abrasivesintended for hand use rather than power operation. Further, the bondedabrasives could be used with various wheel sizes, arbor openings andcombinations of sol-gel abrasives with other abrasive grains such asalumina zirconia grains or with grinding aids other than fluorspar.These and other modifications of the invention will be apparent to thoseskilled in the art.

What is claimed is:
 1. A method of abrading cast iron comprisingproviding a bonded abrasive, said bonded abrasive comprising sol-gelgrain having a grit size of up to about 4 grit in a matrix, and bringingsaid bonded abrasive into frictional contact with cast iron.
 2. Themethod of claim 1, wherein said matrix comprises a phenolic resin. 3.The method of claim 1, wherein said bonded abrasive is in the form of awheel.
 4. The method of claim 1, wherein said frictional contact is asnagging operation.
 5. The method of claim 1, wherein said matrixcomprises a vitreous material.
 6. The method of claim 1 wherein saidbonded abrasive further comprises fused alumina grain.
 7. The method ofclaim 6 wherein said sol-gel alumina comprises about 30 to about 70weight percent of the total abrasive grain in said bonded abrasive.